SpoutDev/Spout — comment on commit d8fd7eff75f5773f57931920bc31bad4cb895e5f
File:  line 0
By Raphfrk:

The basic rule is it depends on what you use the software for. If you use the software as a library, then you don't have to open source you code. However, if you change the source files for the project, then you have to open source the changes.

So, my understanding is that the rule is basically:

If you use our source "as-is" (i.e. as a library), then you don't have open your code
If you create a modified version of our code, then that has to be released.

LGPL is roughly GPL but with the requirement that creating a binary which links to an unchanged version of a project is allowed without transferring the LGPL onto that file.